The present invention relates in general to turbocharged internal combustion engine systems, and more particularly relates to systems in which the exhaust gas from the engine is subjected to an after-treatment process for reducing emissions.
Internal combustion engines are often turbocharged for increasing their power and torque output. One commonly employed type of turbocharger for this purpose is a variable-nozzle exhaust gas turbocharger having a turbine that extracts power from the exhaust gas discharged by the internal combustion engine and drives a compressor that compresses air and delivers the air to the engine intake. The turbine includes a variable-geometry mechanism that can be opened or closed by varying degrees for regulating the flow of exhaust gas into the turbine, thereby regulating how much power the turbine extracts. In this manner, the boost pressure of the air supplied to the intake can be regulated in a desired fashion depending on the engine operating conditions.
There is frequently a need or desire to reduce emissions from internal combustion engines, such as for meeting government-regulated emissions standards. Of special interest is reduction in oxides of nitrogen (NOx) and particulate matter. Various approaches are used to reduce such emissions, including the use of exhaust gas recirculation (EGR) in which a portion of the exhaust gas from the engine is recirculated back to the intake. Another technique for reducing emissions is after-treatment of the exhaust gas using various types of after-treatment devices designed to remove certain substances or convert them into less-offensive forms prior to release of the exhaust gas into the environment.
In some after-treatment devices, the performance of the device can depend on the temperature of the exhaust gas entering the device. In conventional turbocharged engine systems employing a variable-geometry mechanism for the turbine, the temperature of the exhaust gas entering the after-treatment device cannot generally be controlled because the variable-geometry mechanism position is set to achieve a certain boost pressure, and there is no other provision for independently controlling the exhaust gas temperature.